Ever since the advent of digital computers with substantial calculation capabilities, data has been stored externally to the host processor. Even with relay computers of the 1940's, both the program and data storage were separate peripheral data storage systems, each of which provided unit record cards readable by card readers or punched paper tape, which were inputs to the relay computers. With the advent of digital electronic circuits, the demand for greater capability of peripheral data storage became intensified. Magnetic tape and magnetic disks are used as peripheral data storage, wherein data stored on the tapes or disks were read into an electronic memory of the host processor. The addressing of all of the above described peripheral data storage used a single address space for addressing the readers or recorders associated with the peripheral record media. As demand for greater storage capacity continued to increase along with increased computing capabilities, the concept of a data storage hierarchy evolved. In larger data processing installations, the plurality of magnetic tape and magnetic disk recorders and reproducers provides a broad single address base peripheral storage for the data processing installation. Often, along side such peripheral equipment is a true peripheral data storage hierarchy. An example of such a data storage hierarchy is the IBM 3850 Mass Storage System. It is to be understood that other data storage hierarchies also have been employed, but which it is believed did not follow the same addressing and access principles described herein. The IBM 3850 is schematically illustrated in the IBM TECHNICAL DISCLOSURE BULLETIN, article by Blinckenstaff et al., entitled "Multi-level Store Directory Integrity", August 1977, pp. 939-940. In that peripheral data storage hierarchy a lower level of the hierarchy was a magnetic-tape automatic warehouse-type library. The upper level of the peripheral data storage hierarchy consists of a plurality of direct access storage devices (DASD) which is another name for magnetic disk recorders. Addressing in the IBM 3850 was in a single address field, using virtual addressing techniques. Access to all of the data stored in the hierarchy was through the upper level DASD units.
Magnetic tape and magnetic disk peripheral storage subsystems often employ removable media. Such media when removed from the recorder/reproducers were often stored on shelves. The removable media are manually transferred between the shelves and the recorder/reproducers. A mount or demount message is supplied by the using host processor to a console adjacent the recorder/reproducers and convenient to the shelf storage. Operators read the host processor messages and manually carry the removable media from the storage shelves and mount the removable media on either the tape drives or disk drives ( drive is another name for recorder/reproducer). The host processor access to all of the data stored on the removable media is only through tape or disk drives. The addressing is based upon the drive address independent of the access paths which may extend between the host processor and the various drives. Each of the tape reels or disk packs would be assigned a volume serial number, often referred to as VOLSER. In general, the recorded tapes are read in their entirety, in one read operation. Recording on the tape is also effected in a single set of recording operations, i.e. there is no "update in place" on magnetic tape.
With the advent of disk drives, having nonremovable media and an increasing requirement for fast access to peripherally stored data, a random access semiconductor cache was disposed intermediate the disk drive and the host processor. In some of these cached disk drive subsystems, the only access to the data was through the cache. In other of these cached disk drive subsystems, such as shown by Duke et al. in U.S. Pat. No. 4,500,954, the cache could be bypassed for directly accessing the disk drive for recording or reading data. The addressing field of such subsystems was based upon the address space used for accessing data areas of the disk drive. Any data stored in the semiconductor cache is accessed by using the address of the disk drive by checking the cache to see whether or not the data area was allocated in the cache for such disk drive address. Then, if the cache had an allocated area for such address, the host processor accesses the cache allocated area. The disk drives could also be directly accessed whenever the semiconductor cache was out of operation. In some instances, upon a situation wherein the cache did not have allocated space, the cache is bypassed and all of the data would be read from the disk drive directly. Such an arrangement obviated the need for moving the data from the disk drive to the cache, storing it in the cache and then signaling the host processor to fetch the data or write the data to the cache. Accordingly, copies of the data stored in such cached disk drive subsystems were not always acquired from the level having the fastest access. The Duke et. al. U.S. Pat. No. 4,500,954 teaches that recording data into a cached disk drive subsystem may be benefited by not using the cache, i.e. using the cache would degrade performance. Both the cache and disk recorder show host processor access paths; neither the cache nor the disk recorder have separate and independent access paths to the host recorder.
In spite of all of the above described peripheral data storage systems, there is still a need for a peripheral data storage hierarchy that is capable of efficiently storing and retrieving larger amounts of data than previously stored.